Electroluminescent (EL) panels are well known in the art and are used in various applications where a low level of light output is adequate for displays, backlighting or background lighting. Typically, electroluminescent panels are constructed from a sandwich of an opaque planar conductor covered with a phosphorescent insulating compound which in turn is covered with a planar transparent or translucent conductor. An AC voltage potential placed between the two conducting plates causes an excitation of the electrons of the phosphor within the panel, which upon relaxation emit light in the visible spectrum. Electrical potential between the plates is applied as a switched or pulsed AC voltage which upon transition from zero to voltage, voltage to voltage, or voltage to zero levels cause an emission of light from the electroluminescent panel.
Voltage conversion circuits are known in the prior art for driving electroluminescent panels from DC power supply sources. Some prior art power supply sources provide a fixed voltage and fixed waveform to the electroluminescent panel throughout the life of the panel. However, electroluminescent panels age over time. The consequence of the aging is that the panels emit variable or reduced radiation in the visible spectrum as time increases. Variation in visible radiation reduces the commercial or useful life of the panel.
The full effect of power supplies on the rate of panel aging is unknown. The rate of aging is dependent on potential applied across the EL panel, duty cycle of the applied potential, frequency of the applied potential, temperature, humidity, ambient light impinging on the panel, and effective magnitude of the applied potential. However, applied panel driving potentials with small duty cycles and zero effective DC bias are not believed to increase the rate of panel aging. Presently available commercial electroluminescent power supplies or DC to AC inverters do not adequately compensate for this change in panel brightness.
The electroluminescent panel may be modeled electrically as a resistor and capacitor in parallel. The light output of the electroluminescent panel is roughly proportional to the capacitance of the panel. Both resistor and capacitor values change as a function of time, temperature, humidity, applied potential and other factors. Also, it is known that a non-zero DC bias in the driving potential increases the rate of aging of the electroluminescent panel. Prior art power supplies do not actively measure the changing light output or capacitance and compensate for this change with an increased zero bias driving potential. Most of the recent prior art inverters utilize the fact that the electroluminescent panel acts as a parallel capacitor and resistor and hence place this panel in series with an inductor to form a resonant LC circuit. The inverter power supply then drives the panel at resonance. The combination of the passive compensation technique and the power supplies of the prior art inverters results in a distorted sinusoidal driving voltage which can result in a long term effective DC bias placed across the panel which in turn increases aging.